1. Field of the Invention
The present invention relates to a control device and a control method for a power converter to be used by being connected to a field winding type rotary generator-motor including an armature winding and a field winding, the power converter being mounted mainly on a vehicle and operable as an electric motor at the time of starting an engine and as a power generator after the start of the engine.
2. Description of the Related Art
In recent years, for environmental protection and fuel economy improvement, a so-called hybrid car including an engine and another power source such as a rotary generator-motor has been developed for practical use. In this type of vehicle, the engine and another power source are used for different running situations.
Idle reduction, for example, is a technology for reducing unnecessary fuel consumption during idling. In the idle reduction technology, an internal combustion engine is stopped when a vehicle stops at a traffic light or in other situations, and the internal combustion engine is restarted by a rotary generator-motor in response to the detection of a driver's intention to start moving, such as stepping on the accelerator or releasing the brake.
The rotary generator-motor mountable in this type of vehicle is mounted in a manner that can transfer torque to the engine. The rotation speed of the rotary generator-motor thus greatly changes by the driver's accelerator pedal operation and the influence of friction of the engine. As a configuration in which an induced voltage of the rotary generator-motor can be controlled independently of the rotation speed of the engine, there has been adopted a field winding type rotary generator-motor capable of control by a field current instead of using a rotor incorporating a permanent magnet.
In the case of using a rotary generator-motor mounted on a vehicle as a power generator, torque is generated in the engine coupled to the power generator in accordance with the amount of power generation. When the rotation speed is constant, the generated torque increases as the amount of power generation becomes larger. Thus, immediately after the start of the engine by a starter, the amount of power generation is large and the generated torque is also large, and hence the engine may stall immediately after started. In addition, when the generated torque increases during idling due to load fluctuations, the engine may stall because output torque of the engine is low.
To prevent the engine stalling, there has been proposed a field winding type synchronous generator-motor that estimates generated load torque to be applied to the engine at the time of power generation and controls a field winding current of a rotary electric machine so that the generated load torque may be a predetermined value (see, for example, Japanese Patent Application Laid-open No. 2010-81709).
Japanese Patent Application Laid-open No. 2010-81709 discloses no specific conditions as to the selection between generated voltage control and generated torque control. Alternatively, there has been proposed a control device for a power converter, which chooses any one of the generated voltage control and the generated torque control depending on an external command or the load on the power converter so as to suppress fluctuations in generated torque and prevent excessive power generation (see, for example, Japanese Patent Application Laid-open No. 2011-166910).
Generally, the power generator mounted on a vehicle performs generated voltage control so that the voltage of a power source electrically connected to the power generator may be a predetermined value.
In Japanese Patent Application Laid-open No. 2011-166910, however, in the mode of controlling the generated voltage, the generated voltage of the power generator is controlled based on a generated voltage command so as to follow the generated voltage command, while the torque generated by the power generator is not controlled at all and depends on the amount of power generation. As a result, the engine may stall in the state in which the engine output is unstable, such as immediately after the start of the engine, and during idling in which the engine output torque is small.
In the mode of controlling the generated torque, on the other hand, a field current is calculated in accordance with a generated torque command, the rotation speed, and a B-terminal voltage so as to generate a desired torque, and the calculated desired field current is supplied to a field coil to generate the desired torque. The voltage generated by the power generator is therefore not controlled at all and depends on the amount of power generation. Thus, an overvoltage may occur when the generated torque is large.